A heat exchanger used in a refrigeration apparatus, an air-conditioning apparatus, or a heat pump is generally made up of plural fins arranged at intervals and penetrated by heat transfer tubes. Plural tiers of the heat transfer tubes are arranged in a direction intersecting airflow and plural columns of the heat transfer tubes, made up of the plural tiers, are arranged along a direction of the airflow. In a typical configuration, refrigerant flows through the heat transfer tubes, and the heat exchanger makes up a part of a refrigerant circuit.
Some of heat exchangers such as described above use flat tubes with a flat cross section for the heat transfer tubes and have U-bends connected to a pair of coupling portions that are each located in a corresponding one of plural columns of flat tubes that are next to each other or to a pair of coupling portions that are each located in a corresponding one of plural tiers of flat tubes that are next to each other. For example, there is a technique whereby using joints each having a flat cross section at one end, and a circular cross section at the other end, flat tubes are connected with each other via the joints adapted to convert a cross-sectional shape and separate U-bends (hereinafter referred to as “circular-tube U-bends”) made up of circular tubes and adapted to connect circular portions of the joints with each other (see, for example. Patent Literature 1).
Also, there is a technique whereby flat tubes are connected with each other without using a joint such as described above, but using a connecting pipe to which circular tube portions and flat-shaped portions are integrated by forming both end portions of a circular-tube U-bend into a flat shape (see, for example, Patent Literature 2).
Also, there is a technique whereby flat tubes placed at the same height and in different columns are connected with each other using a connecting flat tube that is uniformly flat in cross section (see, for example, Patent Literature 3).
Furthermore, in a heat exchange unit of a double-column structure using flat tubes, a pair of coupling portions that are each located in a corresponding one of flat-tube columns horizontally arranged next to each other are coupled together by U-shaped connecting pipes, the connecting pipes are each created by flattening both ends of a circular tube bent into a U-shape, both end portions of the connecting pipe are made of a clad material with a brazing material bonded to an inner circumferential surface, and the connecting pipes and flat tubes are joined together by brazing (see, for example, Patent Literature 4).